This invention relates in general to trailer brake systems and in particular to an improved electronic brake controller for electrically actuated trailer wheel brakes.
Towed vehicles, such as recreational and utility trailers adapted to be towed by automobiles and small trucks, are commonly provided with electrically actuated wheel brakes. Such brakes generally include a pair of brake shoes associated with each trailer wheel which, when actuated, frictionally engage a brake drum which is attached to the associated wheel. An electromagnet is mounted on one end of a lever to actuate the brake shoes. When an electric current is applied to the electromagnet, the electromagnet is drawn against the rotating brake drum which pivots the lever to actuate the brakes. Typically, the braking force produced by the brake shoes is proportional to the electric current applied to the electromagnet.
Various controllers for electrically actuated brakes are known in the art. For example, a variable resistor, such as a rheostat, can be connected between the towing vehicle power supply and the electromagnets for the towed vehicle wheel brakes. The towing vehicle operator manually adjusts the variable resistor setting to vary the amount of current supplied to the wheel brake electromagnets and thereby controls the amount of braking force developed by the towed vehicle wheel brakes.
Also known in the art are more sophisticated brake controllers which include electronics to automatically supply current to the electromagnets for the towed vehicle wheel brakes when the towing vehicle brakes are applied. Such brake controllers are referred to as electronic brake controllers in the following. Electronic brake controllers typically include a sensing unit which generates a brake control signal corresponding to the desired braking effort. For example, the sensing unit can include a pendulum which is displaced from a rest position when the towing vehicle decelerates and an electronic circuit which generates a brake control signal which is proportional to the pendulum displacement. One such unit is disclosed in U.S. Pat. No. 4,721,344. Alternately, the hydraulic pressure in the towing vehicle's braking system or the pressure applied by the driver's foot to the towing vehicle's brake pedal can be sensed to generate the brake control signal.
Known electronic brake controllers also usually include an analog pulse width modulator which receives the brake control signal from the sensing unit. The pulse width modulator is responsive to the brake control signal to generate an output signal comprising a fixed frequency pulse train. The pulse width modulator varies the duty cycle of the pulse train in proportion to the magnitude of the brake control signal. Thus, the duty cycle of the pulse train corresponds to the amount of braking effort desired.
Electronic brake controllers further include an output stage which is electrically connected to the output of the pulse width modulator. The output stage typically has one or more power Field Effect Transistors (FET's) which are connected between the towing vehicle power supply and electromagnets for the towed vehicle wheel brakes. The power transistors function as an electronic switch for supplying electric current to the towed vehicle wheel brakes. The output stage is responsive to the pulse width modulator output signal to switch the power transistors between conducting, or "on", and non-conducting, or "off", states. As the output transistors are switched between their on and off states in response to the modulator output signal, the brake current is divided into a series of pulses. The power supplied to the towed vehicle wheel brakes and the resulting level of brake application are directly proportional to the duty cycle of the modulator generated output signal.